In the past, semiconductor devices have been provided with electrodes made from materials such as aluminum or gold, but the use of such electrodes is limited due to the relatively low temperature at which these materials form an alloy with silicon. For example, the eutectic point for aluminum and silicon is 350.degree. C. and for gold and silicon is 450.degree. C. Others have used tin oxide for transparent contacts on semiconductor devices, but many applications require higher conductivity than that of tin oxide.
It has been recognized that gallium nitride may be deposited on n-type silicon without including any acceptor impurity in the reacting vapors. In this manner, n-type, conductive gallium nitride can be deposited. When impurities such as zinc, beryllium, lithium or magnesium are added as acceptor impurities, the gallium nitride is insulating. These forms of gallium nitride have been used in electroluminescent devices that include a conductive n-type layer of gallium nitride and an insulating layer of gallium nitride on a sapphire substrate, as in U.S. Pat. No. 3,922,703 invented by Applicant. The properties of n-type gallium nitride disclosed in U.S. Pat. No. 3,922,703 include transparency, high bandgap energy, suitability for growth on sapphire and high conductivity.
Other semiconductor devices have also used gallium nitride in which the substrate is silicon. For example, in U.S. Pat. No. 4,139.858 invented by applicant, transparent, conducting n-type gallium nitride formed a heterotransition when applied to n-type silicon. Radiation in the near UV through the visible spectrum passes through the gallium nitride. However, such radiation results in carrier generation at the surface of the n-type silicon, with loss of the carriers between that surface and a pn junction inside the silicon. As a result, the radiation in the blue to UV wavelength band is not useful for voltage generation in a silicon device having a pn junction.
Other properties of gallium nitride have been recognized by Applicant and others (see "Optical Properties of GaN," RCA REVIEW. Vol. 36, March 1975, by S. Bloom, G. Harbeke and J. I. Pankove). More recent studies of gallium nitride have identified problems in synthesizing cubic gallium nitride and have indicated an expectation that it should be possible to grow cubic gallium nitride on cubic silicon carbide grown on silicon (see "Properties of Gallium Nitride," by J. I. Pankove, Mat. Res. Soc. Svmp. Proc., Vol. 97, 1987.)
Other efforts to fabricate high temperature semiconductor devices have included providing crystalline diamond on silicon carbide. Crystalline diamond has been selected for its large band gap, but problems have been experienced in fabricating a device that takes advantage of this property. For example, there have been problems in making functional heterojunctions using this material, and the conductivity of crystalline diamond has been too low.
Other attempts to fabricate high temperature devices indicate that it is difficult, if not impossible, to predict which, if any, materials having properties that appear suitable for high temperature devices will actually result in an operable high temperature device. For example, zinc oxide, zinc sulphide and silicon carbide have relatively large band gaps, which could indicate that they are suitable for these devices. However, conducting p-type and n-type zinc oxide have not been made and conducting p-type zinc sulphide has not been achieved. Also, the conductivity of silicon carbide is too low to use it as a device electrode.
Others have used gallium arsenide in various semiconductor devices. Gallium arsenide has been passivated by treatment to form a gallium nitride-rich layer on the gallium arsenide.